1858.] HAEKNESS DOLOMITES. 103 



as having been originally deposited in the condition of carbonate of 

 lime and carbonate of magnesia. 



Causes and Date of the Dolomitization. — The whole aspect of these 

 Carboniferons dolomites of Cork and Kilkenny, leads to the inference 

 that certain changes have been effected on previously-existing masses 

 of carbonate of lime ; and the general parallehsm which occurs be- 

 tween these dolomites and the main joints, and also the intimate 

 connexion which exists between them, support the conclusion that 

 this change was produced after the operation of those forces which 

 gave rise to the phenomena of joints. 



With regard to the causes of this subsequent change, we find 

 nothing, in connexion with the carboniferous strata of the county of 

 Cork, to support the inference of Yon Buch, that igneous masses are 

 the means by which dolomitization has been effected ; nor is there 

 any evidence, among these Carboniferous rocks, to show the action 

 of such a degree of heat as usually produces metamorphic phenomena. 

 The general thickness of these magnesian limestones in no way sup- 

 ports the conclusion that the cause producing them operated from 

 below; nor do we meet with anything in connexion with these masses 

 lending any support to such an opinion. On the contrary, we often 

 find the dolomites thickest in their higher position ; and these are 

 circumstances which would rather lead to the conclusion that the 

 cause producing dolomitization was external. 



The only agent we know of capable of acting from above, in such 

 a manner as to produce changes of a character rendering ordinary 

 limestone dolomitic, is sea-water. This, from the abundance of mag- 

 nesian salts which it contains (averaging 8-56 of chloride of magne- 

 sium, and 6-53 of sulphate of magnesia, in 100 parts of solid contents 

 in the water of the German Ocean), is capable, under certain cir- 

 cumstances, of producing important influences on calcareous rocks. 

 According to the observations of E-egnault*, brought under my notice 

 by my friend and colleague Dr. Blyth, the latter salt, sulphate of 

 magnesia, in solution, at a temperature of 200 Celsius, possesses the 

 power of decomposing carbonate of lime, producing sulphate of lime 

 and carbonate of magnesia : and the reverse also takes place, under 

 like circumstances, when sulphate of lime and carbonate of magnesia 

 are brought in contact. Even at ordinary temperatures this latter 

 change takes place. " According to Suckow, the efflorescence of sul- 

 phate of magnesia in the vicinity of Jena depends upon the decom- 

 position of carbonate of magnesia, contained in bitter-spar, by means 

 of gypsum f." 



From these observations of Regnault, the action of sea-water on 

 ordinaiy limestones, at considerable depths, where subjected to great 

 pressure, and at an increased temperature, would exert its influence 

 on these substances in such a manner as to produce important changes 

 on limestones, rendering them dolomitic. This influence of magne- 



* Cours de Chimie, vol. ii. p. 639. 



t Bischof 's Chemical and Physical Geology, vol. i. p. 430. 



